The present invention relates to a vacuum pump to be used to discharge gas from a vacuum chamber installed on equipment for manufacturing semiconductors.
A vacuum pump for generating a vacuum environment is indispensable for use with a CVD apparatus, a dry etching apparatus, a sputtering apparatus, an evaporating apparatus or the like. In recent years, the development of a vacuum pump having a high operational performance is more and more strongly demanded because the process of manufacturing semiconductors must be clean and a high vacuum must be generated.
In order to generate a high vacuum, the semiconductor-manufacturing equipment is provided with an evacuating system comprising a roughing vacuum pump (positive-displacement vacuum pump) and a turbo-molecular pump. The roughing vacuum pump generates a certain degree of vacuum from atmospheric pressure and then, the turbo-molecular pump generates a predetermined high vacuum.
FIG. 11 shows an example of a screw type vacuum pump classified as a conventional positive-displacement vacuum pump (roughing vacuum pump). The screw type vacuum pump comprises a housing 101; a first rotary shaft 102; a second rotary shaft 103; cylindrical rotors 104 and 105 supported by the rotary shafts 102 and 103, respectively; and thread grooves 106 and 107 formed on the peripheral surface of each of the rotors 104 and 105. That is, the first rotary shaft 102 and the second rotary shaft 103, which are parallel with each other, are accommodated in the housing 101. The rotors 104 and 105 are mounted on the first and second rotary shafts 102 and 103, respectively. Thread grooves 106 and 107 are respectively disposed on the rotors 104 and 105 to form the projections and recesses thereof. A space is formed between the thread grooves 106 and 107 by the engagement between the groove of the thread groove 106 and the thread of the thread groove 107 and between the thread of the thread groove 106 and the groove of the thread groove 107. As a result of the rotations of the rotors 104 and 105, the volume of the space changes and thus suction and discharge operations can be performed.
FIG. 12 shows a thread groove type turbo-molecular pump, having a turbine blade, classified as a conventional kinetic vacuum pump. The vacuum pump comprises a housing 151; a cylindrical rotor 152; a turbine blade 153; a thread groove 154; magnetic radial bearings 155a and 155b for supporting a rotary shaft 157; and a magnetic thrust bearing 156. In this construction, the rotor 152 is accommodated in the housing 151. The turbine blade 153 is disposed in an upper portion of the rotor 152. The thread groove 154 is disposed on a lower portion of the rotor 152. Directivity is applied to gas molecules in the motion thereof by the high-speed rotation of the turbine blade 153 (bucket) and the thread groove 154. In this manner, the turbo-molecular pump performs a pumping operation. More specifically, the turbine blade 153 having discharging performance in a molecular flow region of a low pressure is disposed on the suction side of the vacuum pump, and the thread groove 154 having discharging performance in a viscous flow region of a high pressure is disposed on the discharge side. In this manner, a vacuum can be generated over a wide range.
The above-described vacuum pumps and the evacuating system comprising the vacuum pumps have, however, the following drawbacks:
(1) Drawback of roughing vacuum pump (positive-displacement vacuum pump)
In the screw type vacuum pump as shown in FIG. 11, timing gears 110a and 110b are provided to rotate the rotors 104 and 105 synchronously. That is, the rotation of a motor 108 is transmitted to the timing gear 110b, disposed on the shaft of the rotor 105, engaging the timing gear 110a via a driving gear 109a and an intermediate gear 109b. The phase of the rotational angle of each of the rotors 104 and 105 is adjusted by the engagement between the timing gears 110a and 110b. According to this type of vacuum pump, the gears 110a, 110b, 109a, and the intermediate gear 109b are used to transmit the power of the motor 108 and rotate the rotors 104 and 105 synchronously. Consequently, lubricating oil 111 filled in a mechanical operating chamber accommodating the gears must be supplied to the gears. In addition, a mechanical seal 113 is provided between the mechanical-operating chamber and a fluid-operating chamber 112 accommodating the rotors 104 and 105 so as to prevent the lubricating oil from penetrating into the fluid-operating chamber 112.
The screw type vacuum pump, with the above-described construction, having two rotors, has the following drawbacks:
1). Many gears are required to transmit the power of the motor and rotate the rotors synchronously. Therefore, the vacuum pump comprises many parts and thus its construction is complicated. PA1 2). The synchronous rotation of the rotors is effected due to a sliding contact between gears. Therefore, the rotors cannot be rotated at a high speed and the vacuum pump is large. PA1 3). It is necessary to periodically replace the mechanical seal when it becomes worn. That is, the arrangement is not maintenance-free. PA1 4). Since a large sliding torque applied by the mechanical seal, a large mechanical loss occurs.
(2) Drawback of turbo-molecular pump
Similarly to the roughing vacuum pump, the turbo-molecular pump comprises a construction capable of complying with growing demands for the development of a clean semiconductor-manufacturing process. For example, in the thread groove type turbo-molecular pump having the turbine blade as shown in FIG. 12, the magnetic bearings 155a, 155b, and 156 are used instead of ball bearings which require lubrication. In the turbo-molecular pump, a vacuum is formed in the space accommodating the bearings. Normally, in a vacuum atmosphere, it is difficult to lubricate mechanical parts which slidingly contact a counterpart, but the use of the magnetic bearings eliminates the need for lubrication and, the provision of an oil reservoir such as required in the construction comprising the ball bearings. Therefore, the turbo-molecular pump can be installed in an arbitrary posture on a vacuum chamber. However, the cost of the turbo-molecular pump is much higher than the cost of the vacuum pump comprising the ball bearings because it is necessary to provide the shaft of each rotor of the turbo-molecular pump with an electromagnet, a sensor or a controller as described previously.
(3). Drawbacks of evacuating system (i.e. the roughing vacuum pump of (1) above, plus the turbo-molecular pump of +(2) above)
The conventional roughing vacuum pump (positive-displacement vacuum pump) discharges gas in a viscous flow region, the pressure of which is in the vicinity of atmospheric pressure, but the degree of vacuum obtained by the roughing vacuum pump is not greater than approximately 10.sup.-1 Pa. The conventional turbo-molecular pump generates a degree of vacuum as great as approximately 10.sup.-8 Pa, but is incapable of discharging gas in the viscous flow region, the pressure of which is in the vicinity of atmospheric pressure. In order to overcome this drawback, a vacuum environment is generated in a range of 10.sup.0 to 10.sup.-1 Pa by the roughing vacuum pump, for example, the above-described screw pump. Thereafter, a predetermined degree of vacuum is generated by the turbo-molecular pump.
With the progress in composite processes for manufacturing semiconductors in recent years, multi-chamber systems have been widely adopted as equipment for manufacturing semiconductors so as to evacuate a plurality of vacuum chambers independently of each other. An evacuating system comprising the roughing vacuum pump and the turbo-molecular pump is required for each chamber in order to adopt the multi-chamber system. But the provision of the evacuating system for each chamber results in a large and complicated evacuating apparatus.
In order to overcome the above-described drawbacks (1) through (3), one of the present inventors has proposed a vacuum pump in U.S. patent application Ser. No. 738,902 which is pending. The vacuum pump comprises a positive-displacement vacuum pump structure section comprising a plurality of rotors and a kinetic vacuum pump structure section disposed on the shaft of one of the rotors. A plurality of shafts of the rotors is electronically controlled so that the shafts rotate synchronously. In this manner, the vacuum pump can generate a vacuum over a wide range from atmospheric pressure to a high vacuum.